This invention relates to cathodic protection systems and, more particularly, to cathodic protection systems for the internal surfaces of pipes.
Protection against the corrosion of metals, for example, iron and steel, has been provided successfully and at relatively low cost by cathodic protection either by the use of sacrificial anodes or by using permanent anodes coupled to one or more potential sources. These systems overcome and reverse the natural potential difference which is set up whenever a metal is immersed in an electrolyte such as saltwater. Apparatus and systems have evolved which satisfactorily protect ships, liquid-containing tanks, bridges and other structures, and the internal and external surfaces of ducts and pipelines.
Wherever an electrolyte such as saltwater is passed through pipelines, the problem of internal protection is acute. To give reasonable installed life of the pipelines under these circumstances it has been necessary to make the pipes substantially thicker than strength considerations would otherwise call for in order to avoid frequent and costly replacement. Since the pipes and the labor costs of replacing the pipes are costly, the need for providing effective and inexpensive internal protection is fully apparent.
Cathodic protection is accomplished by causing a flow of direct current (DC) between an electrode (called the anode) and the structure (called the cathode), e.g., the pipe. The direct current causes the surface of the structure (pipe) to become polarized, thus stopping or reducing corrosion.
In an impressed current cathodic protection system, a rectifier normally converts AC to DC and supplies the current. Typically, the anode is a relatively inert material that can transfer the current to the liquid in the pipe, which is the electrolyte. In a galvanic system, the anode is an electrochemically active metal compared to the cathode, and the current is a natural occurrence of connecting the anode and cathode together.
The chemical reaction at the anode is metal oxidation, with oxygen, or chlorine evolution. Both ions and electrons are formed at the anode. Ions generated by the reaction flow to the cathode via the electrolyte. The electrons flow to the cathode via a connection. Reduction occurs at the cathode, which consumes the electrons. Three common cathodic reactions in cathodic protection are as follows:O2+2H2O+4e→4OH−O2+4H++4e→2H2O2H++2e→H2 
In all cases, both electron and ionic currents are involved in cathodic protection. That is, both a continuous common electrolyte and a metallic connection between the anode and cathode are required. The metallic connection is provided through the rectifier. The saltwater or other conductive liquid provides the common electrolyte.
Common anode materials include pure metals, alloyed metals, platinum coated valve metals, valve metals having electrochemically active coatings, and certain ceramic materials.
Generally, the internal surfaces of pipelines have been protected in the past by probe anodes or sacrificial linear anodes. Probe anodes are placed into the pipeline at intervals along the pipeline. The probe anode consists of a rigid anode mounted in a fitting that is inserted into the pipeline, generally through a pressure-tight fitting. The probe anode is generally placed perpendicular to the axis of the pipeline and is of no greater length than the diameter of the pipeline. Probe anodes have several disadvantages. First, each probe anode can provide protection for only about four to eight pipe diameters along the length of the pipeline. It would be beneficial to have a system that is continuous and provides uniform protection along the entire length of the pipeline. Second, the probe anode must be installed at regular intervals. This is relatively costly and may affect the integrity of the pipeline. Additionally, the use of a probe anode system may not be possible with underground pipelines. It would be beneficial to provide a system that can cover long distances, for example, distances of five hundred feet or more. Finally, probe anode systems do not provide uniform current distribution. It would be beneficial to provide more uniform current distribution. In the present invention, there is less risk of hydrogen embrittlement of the pipeline metal and less risk of damage to internal coatings.
Sacrificial anodes also have several disadvantages. A sacrificial anode system consists of sacrificial anodes placed inside of the pipeline. The anode material is generally magnesium, zinc, or aluminum alloy. The anode can be in ribbon form of approximately one inch diameter or in block form. The anodes typically contain a steel core and are bolted or welded to the metal pipeline.
The anodes operate via the difference in potential of the metals. For example, zinc has an open circuit potential to a copper—copper sulfate reference electrode of −1.10 VDC, whereas steel would have a potential of −0.55 VDC. The driving voltage would be slightly higher with magnesium and some aluminum alloys. The system is self regulating and, once the anode material is used, the entire system must be replaced.
Again, a sacrificial anode system has several disadvantages. First, it is of relatively high cost. Second, the life of the anodes is limited. Third, the sacrificial anode system is relatively heavy. For example, a sacrificial anode may be about one pound per linear foot or more. Fourth, a sacrificial anode may not have sufficient driving voltage to produce a required DC voltage. With an impressed current system, a transformer rectifier can be provided that will produce much higher driving voltages. Fifth, a sacrificial anode may put metals such as zinc into the liquid stream. Finally, replacement of anodes is difficult.
The present invention improves or corrects these deficiencies.
Several patents are now discussed as general background information. U.S. Pat. No. 6,238,545 (Allebach et al.) discloses an anode embedded in an electrolyte layer applied to the surface of a pipe section to provide an ionic conductive path between the anode and the structure to supply cathodic protection to the structure where the natural environment may not provide a continuous electrolyte. This design protects the exterior of pipes.
U.S. Pat. No. 4,140,614 (McKie) teaches an anode arrangement for use for the internal cathodic protection of a pipe. Here, a hollow anode carrier is connected along the length of the pipe. The anode is offset, but communicates with the channel defined by the pipe so that the anode lies within the recess, but does not obstruct flow in the pipe.
The references cited herein are incorporated herein by reference in their entireties.